1. Field of the Invention
This invention relates to the removal of solids from an organic liquid by using electrical fields. The invention more particularly relates to the removal from organic liquids of solids by their induced adherence upon a particulate bed under the action of d.c. electrical fields.
2. Description of the Prior Art
Non-petroleum organic liquids include fats, vegetable and animal oils which may be used for human consumption, but are not necessarily limited to such utilization. These organic liquids can be purified by removing solids to improve chemical properties, color and visual appearance, and for other reasons. In one example, the hydrogenation of edible oils for preparing more suitable products for human consumption has been known for nearly one hundred years. The modern hydrogenation process for edible oils originated in research work conducted at the turn of this century. In this process, the edible oils, such as cottonseed, soybean, and corn oil, are placed within a reaction vessel (commonly termed a "converter") and brought into contact with hydrogen at elevated temperature and pressure in the presence of a small amount of metal hydrogenation catalyst. For example, the catalyst is usually present in small amounts which may range from 0.01% to about 0.5% by weight based upon the total weight of the edible oils subjected to hydrogenation. Various types of hydrogenation catalysts, such as copper chromite, are known for providing the reaction between hydrogen and the edible oil. For example, one commercial hydrogenation catalyst includes the metal nickel as the principal catalytic agent, but it also may have minor amounts of copper, alumina, or other materials. The metal hydrogenation catalysts are employed principally in a finely defined divided form and are prepared by special methods. Commonly, the nickel metal is placed upon a finely divided, highly porous, inert refractory material, such as diatomaceous earth, or other highly siliceous material. The catalyst is suspended in the edible oils during the hydrogenation process as oil-coated inert solids, which may adsorb soaps or other impurities found in the oil. After the hydrogenation reaction is completed to the desired degree, the hydrogenation reaction materials are removed from the converter. Then, these materials are passed through a filtration system for removing the inorganic solids from the hydrogenated edible oil product.
Various inorganic materials are added to the hydrogenated edible oil product to enhance its filterability. Filteraids usually are employed to promote the product filtration procedure. Various types of relatively sophisticated and expensive filtration equipment are employed. Generally, pressurized filter press assemblies are used, in parallel flow arrangements, to pass the product through a multitude of filter elements which may comprise screen supports carrying paper, canvas or other types of filter medium. These filter elements may be precoated with some type of diatomaceous earth or filteraid to improve oil filterability. The hydrogenated edible oil product is passed through these filters to remove as much as possible of the hydrogenation catalyst material and other inorganic solids materials. However, the filters cannot remove substantially all of the inorganic solids. Additionally, the filters pass decreasing amounts of inorganic solids as the filtration procedure progresses towards an ultimate removal level.
The edible oil industry employs two basic tests to determine the effectiveness of filtration on a hydrogenated edible oil product. In one test, a pound of the edible oil is passed through a filter disc at regulated physical conditions of temperature and time. The filter disc retains impurities above a certain size magnitude leaving a "dark spot" which is compared to a standard set of filter discs. The test is known as the "Filter Disc Impurities Test." The standard discs are numbered 1 through 10, with number 10 being that disc which shows no change in "color" over the unused filter disc. Another test is the analysis of the edible oil for nickel content. The filtration procedure produces an edible oil with from one to several parts per million (ppm) of nickel content. For consumer acceptance and long term storage and other reasons, the nickel content is preferred to be less than 1 ppm of nickel.
Mechanical filtration equipment employed in the edible oil industry usually passes some solids during the filtration procedure. As an example, the edible oil product has a color of relatively low value, e.g., 4, at the beginning of filtration, but then its color test rapidly improves towards a filter disc color of 9 or better. For this reason, the filter equipment cannot produce throughout the edible oil filtered product a filter disc color of 9 or better.
The filtered edible oil, at elevated temperatures, is subjected to additional treating steps which may include treatment with bleaching earth, phosphoric or citric acid or other metal scavengers, with the addition of filteraids, so that the finely divided residual inorganic catalytic and other solids are removed. The edible oil accepted by consumers must have a color of 9 or better. Thus, filtration or other color improvement procedures are employed with each edible oil subjected to the hydrogenation. Substantial time elapses in these procedures and subjects the edible oils to aging. The procedures are practiced until the edible oil has a filter disc color of 9 or better, and residual hydrogenation catalyst solids, and possible colloidal nickel metal, is at an acceptable low value.
Nearly forty years ago, it was proposed to purify edible oils by adding a small amount of a loader-type solids, such as "10% activated clay" in a finely divided admixture to the edible oil. This mixture was then flowed horizontally in a zigzag path between closely spaced electrodes which were energized to elevated potentials. The impurities and the loader solids were to migrate to one or the other electrodes for their removal from the edible oil. Accumulated solids might "drop off" after reaching a certain thickness on the electrodes or they could be removed by interrupting or reversing the current. Mechanical removal of the solids from the electrodes could also be used. Unfortunately, the unpacked dielectric zone between the electrodes will not remove substantially all of the impurities from the edible oil. One reason for this result may be that the electric field cannot be made of sufficient intensity even with closely spaced electrodes that substantially all of the inorganic solids could be removed from the edible oils. Some solids pass through the spaces between the electrodes without being attracted to them for removal from the edible oil.
It has been proposed for nearly as many years to employ electrofiltration systems for removing inorganic and organic solids from dielectric natural and petroleum oils. For this purpose, the dielectric oil carrying the solids is passed through a particulate material within an elevated d.c. electric potential field. The d.c. field can have potentials of between 5 and 200 kilovolts established across the bed. The bed might be composed of particles of materials such as furnace slag, sand, gravel, limestone, crushed glass, glass bead, ceramics, palletized clay, and like solid materials. Various types of electrode arrangements have been advocated for providing the high voltage d.c. electrical field for these electrofilter devices. The electrofilter devices, which have been known in the past, have been highly effective for removing inorganic solids from dielectric liquids such as the hydrocarbon products of crude oil refining. Unfortunately, these electrofilters are very effective in attracting solids to the particulate bed materials. These removed solids adhere very strongly to the bed material even with the d.c. electric field removed from the electrofilter device. Once the bed is subject to a solids-fillup, substantial changes in electrical conductivity occur which induce arcing in many instances. Arcing can produce a sudden release of solids into the dielectric liquid- Then, the bed must be cleaned before being reused.
The tenacious adherence of inorganic solids upon the particulate bed of electrofilters has required special techniques for regeneration. Example may be taken to U.S. Pat. Nos. 3,394,067, 3,799,855, 3,799,856 and 3,799,857 for illustrations of various types of practical and commercial bed cleanup systems. These patents describe cleanup procedures which produce a very high mechanical agitation between the particles of the electrofilter bed for mechanically removing adhering inorganic solids. Naturally, high levels of mechanical contact between these particles cause abrasion and could result in a small amount of abraded particle material being released from the bed. As a result, the bed would have to be scrupulously cleaned of the small fragmented portions of the particulate material unless these residues were not critical to the dielectric liquid being treated. In the electrofiltration purification of hydrocarbon materials, such as distillates and residual streams or inorganic materials, a small amount of residue from abraded particulate bed material would not be a problem. However, these abraded materials are intolerable in edible oils.
The prior art has included proposals to employ electrofiltration for purification of various types of dielectric organic liquids for about four decades. However, no electrofilter device has produced the purification of organic liquids which contain finely-divided inorganic and organic solids for several important reasons. In the first instance, the electrofilter equipment must effect complete removal from solids without either an unacceptable pressure buildup or deleterious change in electrical characteristics inducing arcing, shorting and like problems. In the second instance, the electrofilter must be capable of handling effectively the organic liquid stream containing widely varying amounts of solids. In the third instance, the electrofilter system must contain a particulate bed material of special characteristics so that (1) the organic liquid product is produced with extremely low amount of residual solids, without undue pressure buildup or arcing, (2) the particulate bed material is readily cleaned of adhering solids, and (3) no abrasion or loss of the particulate bed material itself occurs to contaminate the organic liquid. The foregoing reasons summarize the incapability of prior art electrofiltration system in being used in the purification of organic liquids containing finely divided solids.
The present invention is an electrofiltration system for purifying organic liquids by removal of finely-divided solids without the foregoing recited problems of prior art electrofilters. In particular, the present invention is a novel electrofilter, electrofiltration process, and organic liquid product. A selected material in the bed of the electrofilter system produces a substantially complete removal of the solids, irrespective of the solids loading content, and without the electrofilter system having any appreciable pressure buildup or electrical arcing in the bed. The bed is chemically inert, and no contamination of the organic liquid stream occurs in use. The cleaning of the bed of accumulated solids is not complicated and produces an environment for continuous and repeated electrofiltration for producing a product organic liquid equal in physical and chemical characteristics to commercial product prepared by extended time, multistep procedures. Additional features of the present invention will be appreciated from the following description.